require 'singleton' require 'active_support/core_ext/array/wrap' require 'active_support/core_ext/module/aliasing' require 'active_support/core_ext/module/remove_method' require 'active_support/core_ext/string/inflections' module ActiveModel module Observing extend ActiveSupport::Concern module ClassMethods # == Active Model Observers Activation # # Activates the observers assigned. Examples: # # # Calls PersonObserver.instance # ActiveRecord::Base.observers = :person_observer # # # Calls Cacher.instance and GarbageCollector.instance # ActiveRecord::Base.observers = :cacher, :garbage_collector # # # Same as above, just using explicit class references # ActiveRecord::Base.observers = Cacher, GarbageCollector # # Note: Setting this does not instantiate the observers yet. # +instantiate_observers+ is called during startup, and before # each development request. def observers=(*values) @observers = values.flatten end # Gets the current observers. def observers @observers ||= [] end # Instantiate the global Active Record observers. def instantiate_observers observers.each { |o| instantiate_observer(o) } end def add_observer(observer) unless observer.respond_to? :update raise ArgumentError, "observer needs to respond to `update'" end @observer_instances ||= [] @observer_instances << observer end def notify_observers(*arg) if defined? @observer_instances for observer in @observer_instances observer.update(*arg) end end end def count_observers @observer_instances.size end protected def instantiate_observer(observer) #:nodoc: # string/symbol if observer.respond_to?(:to_sym) observer = observer.to_s.camelize.constantize.instance elsif observer.respond_to?(:instance) observer.instance else raise ArgumentError, "#{observer} must be a lowercase, underscored class name (or an instance of the class itself) responding to the instance method. Example: Person.observers = :big_brother # calls BigBrother.instance" end end # Notify observers when the observed class is subclassed. def inherited(subclass) super notify_observers :observed_class_inherited, subclass end end private # Fires notifications to model's observers # # def save # notify_observers(:before_save) # ... # notify_observers(:after_save) # end def notify_observers(method) self.class.notify_observers(method, self) end end # == Active Model Observers # # Observer classes respond to lifecycle callbacks to implement trigger-like # behavior outside the original class. This is a great way to reduce the # clutter that normally comes when the model class is burdened with # functionality that doesn't pertain to the core responsibility of the # class. Example: # # class CommentObserver < ActiveModel::Observer # def after_save(comment) # Notifications.deliver_comment("admin@do.com", "New comment was posted", comment) # end # end # # This Observer sends an email when a Comment#save is finished. # # class ContactObserver < ActiveModel::Observer # def after_create(contact) # contact.logger.info('New contact added!') # end # # def after_destroy(contact) # contact.logger.warn("Contact with an id of #{contact.id} was destroyed!") # end # end # # This Observer uses logger to log when specific callbacks are triggered. # # == Observing a class that can't be inferred # # Observers will by default be mapped to the class with which they share a # name. So CommentObserver will be tied to observing Comment, ProductManagerObserver # to ProductManager, and so on. If you want to name your observer differently than # the class you're interested in observing, you can use the Observer.observe class # method which takes either the concrete class (Product) or a symbol for that # class (:product): # # class AuditObserver < ActiveModel::Observer # observe :account # # def after_update(account) # AuditTrail.new(account, "UPDATED") # end # end # # If the audit observer needs to watch more than one kind of object, this can be # specified with multiple arguments: # # class AuditObserver < ActiveModel::Observer # observe :account, :balance # # def after_update(record) # AuditTrail.new(record, "UPDATED") # end # end # # The AuditObserver will now act on both updates to Account and Balance by treating # them both as records. # class Observer include Singleton class << self # Attaches the observer to the supplied model classes. def observe(*models) models.flatten! models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model } remove_possible_method(:observed_classes) define_method(:observed_classes) { models } end # Returns an array of Classes to observe. # # You can override this instead of using the +observe+ helper. # # class AuditObserver < ActiveModel::Observer # def self.observed_classes # [Account, Balance] # end # end def observed_classes Array.wrap(observed_class) end # The class observed by default is inferred from the observer's class name: # assert_equal Person, PersonObserver.observed_class def observed_class if observed_class_name = name[/(.*)Observer/, 1] observed_class_name.constantize else nil end end end # Start observing the declared classes and their subclasses. def initialize observed_classes.each { |klass| add_observer!(klass) } end def observed_classes #:nodoc: self.class.observed_classes end # Send observed_method(object) if the method exists. def update(observed_method, object) #:nodoc: send(observed_method, object) if respond_to?(observed_method) end # Special method sent by the observed class when it is inherited. # Passes the new subclass. def observed_class_inherited(subclass) #:nodoc: self.class.observe(observed_classes + [subclass]) add_observer!(subclass) end protected def add_observer!(klass) #:nodoc: klass.add_observer(self) end end end